1. FIELD OF THE INVENTION
The present invention relates to an improved self-compensating device for a drum brake and more particularly to a self-compensating device for a drum brake of the type including a first member and a second member together defining a hydraulic chamber in a wheel cylinder, said first and second members being arranged slidably but not rotatably relative to one another so that brake shoes are actuated by displacement of both the members away from one another which is initiated by hydraulic pressure introduced into the hydraulic chamber, wherein wearing of the brake shoes is automatically compensated to maintain a constant working clearance for the brake shoes, reliable braking is always assured by a predetermined amount of actuation of a brake actuating means such as brake pedal or the like, and inspection and maintenance are easily carried out for the drum brake.
2. DESCRIPTION OF THE PRIOR ART
As is well known, parts and components in a conventional drum brake are usually deformed elastically when heavy braking is initiated. Apparently, the elastic deformation thus developed is recognized as wearing of braking elements. It has been found out that when a conventional self-compensating device is operated in response to the elastic deformation after the latter has been developed, excessive compensation (over-adjustment) takes place, resulting in an occurance of so-called dragging phenomenon due to extremely reduced working clearance produced on release from the braked state. To obviate the foregoing problem there have been proposed a variety of self-compensating devices which are equipped with a mechanism for inhibiting excessive compensation. However, it has been pointed out as a drawback inherent to these proposed devices that the drum brake cannot be compactly constructed because the mechanism for inhibiting excessive compensation is disposed outside the hydraulic chamber of the wheel cylinder.
Further, it has been found that the drum brake with a self-compensation device incorporated therein has to encounter another problem that when the used brake shoes are to be replaced with new ones, removal of the brake drum in the axial direction is carried out only with much difficulties due to interference of the brake shoes with roughness on the inner surface of the brake drum because the aforesaid roughness is developed along the inner surface of the brake drum by its frictional sliding movements with respect to the brake shoes during long term operation of the drum brake and the brake shoes are displaced in the direction of expansion by means of the self-compensating device in response to their wearing for the purpose of compensating the latter.
To obviate the last mentioned problem there have been also proposed a plurality of improved drum brakes. The drum brake as disclosed in Japanese Published Utility Model NO. 35,274/79 is a typical one among them. Specifically, the drum brake proposed therein is the type in which a parking brake lever extending through a back plate is mechanically associated with a brake shoe at the innermost end thereof and further a stopper member for holding the parking brake lever in the inoperative position is fixedly secured to the back plate, the brake being improved in that an abutment piece is detachably mounted on the parking brake lever; and a stopper wall for holding the parking brake lever in the normal inoperative position with the aid of said abutment piece and a recessed part for allowing the parking brake lever to be retracted rearward from the inoperative position when releasing the abutment piece are provided on the stopper lever. Since the drum brake is constructed in that way, it is assured that the brake drum can be easily removed in the axial direction without hindrance due to roughness developed during operation of the drum brake on the inner surface of the brake drum, because a clearance between the brake drum and the brake shoe is amply enlarged by dismounting the abutment piece and thereby contracting the brake shoe from the normal inoperative position.
However, the contracting means for contracting the brake shoes to facilitate removal of the brake drum can be applied only to a drum brake of the type including the aforesaid parking brake lever and therefore the improved drum brake is available within a limited extent of application.